Field of the Invention
The present invention relates generally to weather-resistant pearlescent pigments.
Description of Related Art
Pearlescent pigments which contain titanium dioxide in the top layer or which are made up of particulate TiO2 have a degree of photocatalytic activity. If UV light acts on a pearlescent pigment in the presence of water and oxygen, the UV activity of the pearlescent pigment can trigger an accelerated decomposition of organic compounds, e.g. of a binder matrix. Even the proportion of UV contained in daylight can cause this reaction, i.e. for applications such as automotive coatings which are directly exposed to the weather, specially stabilized pearlescent pigments must be used. In order to counteract this adverse effect for external application, pearlescent pigments can be provided with various protective coatings to reduce the photoactivity. Starting from aqueous metal salt solutions, poorly soluble compounds are usually precipitated as metal oxides onto the surface of the pigments. Two different metal oxides are predominantly used here. In order to promote the compatibility of the pigments with different coatings, but in particular with the more environmentally sustainable water-based systems, as a rule an additional organic modification of the top layer is also applied, e.g. by means of silanes.
According to the teaching of EP 0 632 109 A1 a three-layered protective layer is applied to a platelet-shaped substrate coated with metal oxides. In a first step an SiO2 layer is applied, in a second step a hydroxide or hydrated oxide of cerium, aluminum or zirconium is applied and in a third step at least one hydroxide or hydrated oxide of cerium, aluminum or zirconium and an organic coupling reagent are applied. This three-layered structure is disadvantageously very laborious and correspondingly cost-intensive to produce. In addition the coupling reagents have to be hydrolyzed before binding to the pigment surface, wherein, however, according to the teaching of EP 0 888 410 B1, only a maximum of 60% of the added coupling reagents can be bound to the pigment surface.
EP 0 888 410 B1 discloses modified pearlescent pigments based on a platelet-shaped substrate coated with metal oxides. According to the teaching of EP 0 888 410 B1, the top layer consists of at least two oxides, a mixture of oxides, or mixed oxides of silicon dioxide, aluminum oxide, cerium oxide, titanium dioxide or zirconium dioxide and a water-based oligomeric silane.
EP 1 682 622 B1 also discloses a top layer made of two metal oxides, wherein here first a cerium oxide layer must be precipitated and subsequently an SiO2 layer. Silanes are likewise predominantly used as coupling reagents.
EP 0 881 998 B1 discloses weather-resistant pearlescent pigments with a top layer either made of aluminum oxide or again of a two-layered structure made of aluminum oxide and cerium oxide as well as silanes as coupling reagents.
EP 0 141 174 B1 discloses weather-resistant pearlescent pigments with a top layer which contains cerium hydroxide. In this document it is proposed that this top layer be supplemented by a silicate layer and preferably by further oxide layers, such as aluminum oxide or zinc oxide, in order to guarantee a better binding of polymeric siloxanes which can function as coupling agents.
DE 103 48 174 A1 discloses weather-resistant pearlescent pigments in the case of which, i.a., a layer of tin dioxide in the top layer guarantees the condensation water stability.
In the above-named state of the art, the UV activity of the highly refractive TiO2 layer is usually suppressed by at least two different oxide layers or a mixed layer of two oxides.
In EP 1 084 198 B1 effect pigments are described which, because of their surface modification with reactive orientation agents, have very good adhesion to the base coat. However, EP 1 084 198 B1 does not disclose any weather- and UV-resistant pearlescent pigments.
The available weather-resistant coatings reach their limits when it comes to the coating of optically very high-quality pearlescent pigments. When synthetic substrates such as for example glass flakes, i.e. glass platelets, or synthetic mica are used, pearlescent pigments are obtained which are very pure in terms of color. Any subsequent coating in the form of a top layer for weather-resistance can easily lead to undesired color changes in the optical impression of the pearlescent pigment. These can have various causes: the coating can have an inherent color, which is the case e.g. with cerium oxides. When very thick top layers are used, an optical shift can take place as a result of interference, for example when using ZrO2, or there can be a loss of gloss, for example when using SiO2. In particular in the case of pearlescent pigments with very thick TiO2 layers, which lead to interferences in the region of the third order, and very fine, i.e. very thin substrates, the photocatalytic activity of the TiO2 layer is particularly pronounced and distortion occurs with and in the case of the known stabilising systems.
On the other hand, when using a top layer which is very thin as a whole to prevent optical distortion, this must still guarantee the weather-resistance of the pearlescent pigment very effectively.